Ab Initio Study of the Barrier to Planarity of Cyclobutane, Silacyclobutane and Germacyclobutane*

The structures and barriers to planarity of cyclobutane (CB), silacylobutane (SiCB) and germacyclobutane (GeCB) have been computed using MP2, CCSD, MP4(STDQ) and CCSD(T) ab initio methods and the cc-pVTZ basis set in each case. These results were compared to experimental values previously reported. The reported experimental barrier to planarity of CB ranges from 448 to 518 cm$^{\mathrm{-1}}$. The CCSD/cc-pVTZ calculation gives a value of 586 cm$^{\mathrm{-1}}$ for CB, which is in closer agreement than the other ab initio results. For SiCB the reported experimental barrier is 440 cm$^{\mathrm{-1}}$. Our calculated CCSD/cc-pVTZ barrier is 472 cm$^{\mathrm{-1}}$, in very good agreement with the experimental value, but the other methods do more poorly. The experimental value of GeCB has not been determined, but we have calculated its CCSD/cc-pVTZ value to be 409 cm$^{\mathrm{-1}}$, which we consider to be a reasonable approximation. The trend to lower barrier heights from CB to SiCB to GeCB is expected since the torsional forces involving SiH$_{\mathrm{2}}$-CH$_{\mathrm{2}}$ interactions or GeH$_{\mathrm{2}}$-CH$_{\mathrm{2\thinspace }}$interactions are less than the CH$_{\mathrm{2}}$-CH$_{\mathrm{2}}$ interactions of CB.